DESCRIPTION: The aim of our research program is to better understand the mechanisms involved in language acquisition by examining the differing effects of nature and nurture on this process. Toward this aim, we ask how the specific course of language acquisition is affected by differences in environment. This project investigates the nature of first language acquisition under delayed input as compared with native input. The motivation for this project is the search for understanding the nature of the cognitive mechanisms of language development, and how they might change over time. Existing longitudinal case study data from four deaf children acquiring American Sign Language (ASL) as a first language with input from birth, and two with input beginning only at 6 years of age, are examined to address specific research hypotheses about the nature of a sensitive period for language acquisition. There has long been speculation that linguistic input must be received within a certain critical, or sensitive period, in order for language development to occur within normal limits. This is the first study to examine this proposal through detailed analysis of longitudinal data from children whose input begins only well into schoolage. RELEVANCE This project investigates the nature of a possible critical period for language acquisition, by studying the acquisition of American Sign Language by deaf children who are exposed to it from birth, as compared with deaf children with no language input until the age of 6 years. Understanding both the normal course of development of ASL and what happens when language input is considerably delayed will help us to understand how language develops in the minds of all children. The project also supplies valuable information on sign language development, needed to understand normal and impaired development by deaf children.